GAS: Fix `abort' expansion in write.c

Remove an internal diagnostic regression introduced with the inclusion of
"libbfd.h" from write.c, added with:

commit e7ff5c73
Author: Alan Modra <amodra@gmail.com>
Date:   Fri Feb 16 03:40:17 2007 +0000

That change made "libbfd.h" override the `abort' definition provided by
"as.h" earlier on, making the message produced by any calls reached from
write.c, which is a part of the GAS proper, look like they came from
BFD, e.g.:

.../gas/testsuite/gas/elf/type.s: Assembler messages:
.../gas/testsuite/gas/elf/type.s:30: Error: symbol type "gnu_unique_object" is supported only by GNU targets
../as-new: BFD (GNU Binutils) 2.26.51.20160628 internal error, aborting at .../gas/write.c:608 in size_seg

../as-new: Please report this bug.

vs:

.../gas/testsuite/gas/elf/type.s: Assembler messages:
.../gas/testsuite/gas/elf/type.s:30: Error: symbol type "gnu_unique_object" is supported only by GNU targets
.../gas/testsuite/gas/elf/type.s: Internal error, aborting at .../gas/write.c:602 in size_seg
Please report this bug.

With the removal of "libbfd.h" restore the latter message format.

	gas/
	* write.c: Remove "libbfd.h" inclusion.